Kailash Sahu (STScI, Baltimore, US)
Studying White dwarfs and Black Holes Through Astrometric Microlensing
Exactly 100 years after the famous 1919 solar eclipse experiment that confirmed Einstein’s general theory of relativity, the nearby single white dwarf Lawd 37 passed very close to a background star in 2019. As Lawd 37 passed by, the background star's position was deflected—an effect known as astrometric microlensing. Measurement of this deflection with HST allowed us to determine the mass of Lawd 37 – the first such mass measurement of a single white dwarf -- as 0.56 +/- 0.08 solar masses. This mass determination provides confirmation of the physics of degenerate matter and lends support to white dwarf evolutionary theory. I will then report our first unambiguous detection and mass measurement of an isolated stellar-mass black hole (BH). We used the Hubble Space Telescope (HST) to carry out precise astrometry of the source star of the long-duration (T ~ 270 days), high-magnification microlensing event OGLE-11-462. Combining HST astrometry, OGLE photometry, and spectroscopic measurements, we obtain a lens mass of 7.1 +/- 1.3 solar mass and a distance of 1.58 +/- 0.18 kpc. We show that the lens emits no detectable light, which confirms its BH nature. Its proper motion is offset from the mean motion of Galactic-disk stars at similar distances by ~45 km/s, suggesting that the BH received a “natal kick'' from its supernova explosion. The current status of our ongoing project to detect such isolated stellar-mass BHs will be discussed.